I have a Li-ion battery that I pulled from a MS Band 2. It comes with a protection chip, which I am having trouble with.

The chip has 4 accessible pins, labeled "P+, K, T, P-". I've tried every combination of those 4 pins across an LED and never got any current. I am able to get power if I connect straight to the battery's terminals (B+, B-). I also discovered that I could get power out of P+ and the battery's negative terminal (B-). I've triple checked my solder, so it isn't a connection issue.

At this point, I don't know how to get power out of the protection circuit safely, nor how to charge the battery. I have yet to try hooking up K or T to an I2C, but I don't think that would help here. I know that T is most likely the temperature resistor, but I don't know how to wire that up. I don't know (and couldn't find any info on) what the K terminal is.

Please help! Thanks.

  • \$\begingroup\$ Welcome to EE.SE! " 4 pins across an LED and never got any current". Please don't troubleshoot with an LED since it brings on many limitations and challanges. Do you have access to a normal multimeter? \$\endgroup\$ – winny Aug 24 '18 at 7:22
  • \$\begingroup\$ Unfortunately not, but I do have an Arduino. Perhaps I can use the analog inputs (0-1023 for 0-5V) as a temporary one? \$\endgroup\$ – Eugene Aug 24 '18 at 15:46
  • \$\begingroup\$ Better than nothing, but you will need a DMM in your life, especially now. \$\endgroup\$ – winny Aug 24 '18 at 15:56
  • \$\begingroup\$ I know :P I might be able to access one in my uni's labs. Any suggestions as to how to use it here? \$\endgroup\$ – Eugene Aug 24 '18 at 18:35
  • \$\begingroup\$ Measure all voltages with reference to P-. \$\endgroup\$ – winny Aug 24 '18 at 18:40

Typically what happens with a battery with protection when overdischarged is that the cell gets disconnected from external terminals, to prevent deeper discharge. Unfortunatley every cell has certain self-discharge, so the cell continues to drop its voltage, and the protection circuit keeps engaging. So the battery measures zero volts externally. However, the protection circuit usually allow the charge in.

You should try to apply maybe 50-100mA of current to P+ and P- terminals, and wait 10-15 minutes. You can use a 5V source and a 50-100 Ohms resistor in series. If the cell is not completely dead, it should slowly charge above 2.5-3V. Then the protection circuit will disengage, and you can try to charge the battery using normal Li-Ion charger. However, the battery capacity can be severely diminished, down to zero. The effect depends on how long the cell was in overdischarged state.

  • \$\begingroup\$ The battery does have charge, though. I can get 2V out of the battery terminals, just not out of the protection circuit. \$\endgroup\$ – Eugene Aug 25 '18 at 16:09
  • \$\begingroup\$ @Eugene, 2V for Li-ion is a deep overdischarge. The protection IC is in disconnect state. With slow charging you should get to 3.3-3.5V, you should be able to do it via battery terminals, or via external terminals, either way. Does it charge above 2 V if you use a 100-Ohm resistor and 5 V supply? \$\endgroup\$ – Ale..chenski Aug 25 '18 at 17:41

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